The present invention relates to systems for monitoring and measuring vibration or mechanical motion of equipment. A sensor is mechanically coupled to the equipment, which sensor is an optical waveguide such as fiber optic cable. The light transmission characteristic of the optical waveguide is altered by the vibration or mechanical force imparted to it from the equipment to which it is coupled.
The use of fiber optic sensors for vibration monitoring or as mechanical motion detectors is set forth in U.S. Pat. No. 3,709,030, and NASA Tech Brief LEW-13219, "Lightweight Optics With Digital Output Fiber Optic Accelerometer". Both of the above teachings utilize the concept of having a light transmissive fiber optic cable free to vibrate or move with the equipment to which it is coupled, and to use an opaque chopper or segmented light detector to sense the movement of the fiber optic cable.
A fiber optic sensor has obvious advantages permitting its introduction into hostile environments, particularly where strong electromagnetic interference is present to generate significant electrical noise levels for most electrical sensing means. A fiber optic sensor is unaffected by such interference. The fiber optic sensors used to date require precision alignment and assembly which is difficult to attain in field equipment. These prior art sensors require high intensity light surce input, which generally means short light source lifetime, which contributes to system unreliability. A vibration sensing system is required to be on line for extended maintenance-free operation such as in a turbine or generator of an electrical power station.
It is well known that when a fiber optic cable is bent beyond a predetermined radius the light transmission characteristics of the cable are adversely affected, and this effect is termed "bending losses". The light directed through a fiber optic cable is normally internally reflected at the core-cladding boundary. When the fiber is bent beyond a critical radius, the light through the cable core strikes the core-cladding boundary at an angle greater than the critical angle, and will not be totally internally reflected, but will be lost through the cladding.